1. The Evolution of Populations

2. Activating Prior Knowledge
If you were in charge of putting together the Olympic dream team, what are the chances you could do it with only 10 people trying out?
What if you had 30?

3. Genetic Variation within Populations
In a population there are a wide range of phenotypes.
Greater range of phenotypes= greater genetic variation
Greater range of phenotypes= better chances of survival in changing environments

4. Genetic Variation within Populations
Genetic variation is kept in a population’s gene pool.
Gene pool: the combined alleles of all of the individuals in a population.
Different mixes of alleles in a gene pool can be made when organisms mate and have offspring.
Allele frequency: a measure of how common a certain allele is in the population.

5. Genetic Variation within Populations
Let’s try calculating the allele frequency of this population:
Figure out how many Pink (P) and how many black (p). Then divide each by the total number of alleles.

6. Genetic Variation within Populations
Genetic variation comes from 2 things: Mutations and Recombinations
Mutations: Random change in the DNA of a gene. New alleles from mutations occur frequently in gene pools.
Recombinations: New allele combos form in offpsring. Occurs during meiosis. Parent alleles are rearranged in new combinations during gamete formation.

7. Natural Selection in Populations
Normal distribution: type of distribution in which the highest frequency is near the mean value and decreases toward each extreme end of the range.

8. Natural Selection in Populations
Phenotypes in the middle of the range tend to be the most common.
Environmental changes can occur and a certain phenotype may be an advantage.
Alleles associated with favorable phenotypes increase in frequency.

9. Natural Selection in Populations
Microevolution: the observable change in the allele frequencies of a population over time.
Occurs on a small scale within a single population.
Natural selection can lead to microevolution in one of 3 ways: directional, stabilizing, and disruptive selection

10. Natural Selection in Populations
Directional selection: favors phenotypes at one extreme of a trait’s range.
Causes a shift in a population’s phenotypic distribution.
A rare phenotype becomes more common.

11. Natural Selection in Populations
Stabilizing selection: the intermediate phenotype is favored and becomes more common in the population.
Can decrease genetic variation as in the gall fly.

12. Natural Selection in Populations
Disruptive selection: occurs when both extreme phenotypes are favored, while individuals with intermediate phenotypes are selected against by something in nature.

13. Other Mechanisms of Evolution
When an organism joins a new population and reproduces, it’s alleles become part of that population’s gene pool.
It also removes those alleles from the old gene pool.
Gene flow: the movement of alleles from one population to another.
Increases the genetic variation of the new population.
Less gene flow between 2 populations= more genetically different.
Lack of gene flow increases the chances of the 2 populations evolving into different species.

14. Other Mechanisms of Evolution
Genetic drift: changes in allele frequencies that are due to chance.
Causes a loss of genetic diversity in a population.
Bottleneck effect: genetic drift that occurs after an event greatly reduces the size of a population.

15. Other Mechanisms of Evolution
Founder effect: genetic drift that occurs after a small number of individuals colonize a new era.
Gene pools are very different from those of the larger populations.
Example: Amish of Lancaster, PA. have high rate of Ellis-van Creveld syndrome.

16. Other Mechanisms of Evolution
Genetic drift causes the population to lose genetic variation.
A population is less likely to have individuals that will be able to adapt to a changing environment.
Any alleles that are lethal in homozygous individuals can be carried by heterozygous individuals, and now becomes more common in the gene pool.

17. Other Mechanisms of Evolution
Mating has an important effect on evolution of populations.
Cost of reproduction differs between males and females: Males have lots of sperm, so their value is relatively small. Females have limited eggs, which makes each one more valuable.
This is the reason why females can be more choosy about who they mate with.

18. Other Mechanisms of Evolution
Sexual selection: occurs when certain traits increase mating success. 2 types:
Intrasexual selection involves competition between males. Whoever wins gets the female.
Intersexual selection happens when males display certain traits that attract a female.
Some showy traits are signs of quality and health in males, even though they can attract predators.

19. Hardy-Weinberg Equilibrium
1908- Hardy and Weinberg showed that genotype frequencies in a population stay the same over time as long as certain conditions are met.
These frequencies can be predicted.
They identified 5 conditions that are needed for a population to stay in equilibrium.
Populations that meet these 5 are not evolving. They are in Hardy-Weinberg equilibrium.

20. Hardy-Weinberg Equilibrium
5 conditions needed:
Very large population-no genetic drift can occur
No emigration or immigration-no gene flow can occur
No mutations-no new alleles added to gene pool
Random mating-no sexual selection can occur
No natural selection-all traits must equally aid in survival
Very rarely are all 5 conditions met.
We can predict genotype frequencies of simple systems using the Hardy-Weinberg equation.
If the prediction matches the actual than the population is in Hardy-Weinberg equilibrium, if not it’s evolving.

21. Hardy-Weinberg Equilibrium
Let’s say we have a population of 1000 fish. 640 have forked tails and 360 have smooth tails. (T) is used to show the dominant allele and (t) is used for smooth tales. Find the predicted allele frequency and the predicted genotype frequency.

22. Hardy-Weinberg Equilibrium
5 facotors can lead to evolution:
Genetic Drift-allele frequencies can change due to chance
Gene Flow-movement of alleles change the frequencies
Mutation
Sexual Selection-certain traits may improve success of mating. These alleles become more frequent.
Natural Selection-Certain traits may be an advantage for survival. These alleles increase in frequency.
Evolution is continuous.

23. Speciation Through Isolation
How do we know that a 3ft tall Irish Wolfhound and a 6 in high Chihuahua are the same species?
At what point would the 2 breeds become separate species?

24. Speciation Through Isolation
If gene flow between 2 populations stop  isolated
Gene pools change as they adapt to the environment.
Mutation and genetic drift can also change gene pools.
Over time isolated populations become more and more genetically different.
Individuals of each group may start to look and behave differently from one another.

25. Speciation Through Isolation
Reproductive Isolation: occurs when members of different populations can no longer mate successfully with one another.
Sometimes they physically can’t. Others can’t produce an offspring that survive and reproduce.
Reproductive isolation is the final step of becoming separate species.
Speciation: the rise of 2 or more species from one existing species.

26. Speciation Through Isolation
Populations can become isolated in a few ways.
Behavioral isolation: isolation caused by differences in courtship or mating behaviors.
Example: Different species of fireflies emit light at different times to attract mates of the same species.
Geographic Isolation: physical barriers that divide a population into 2 or more groups.
Example: Isthmus of Panama separated many Marine species.

27. Speciation Through Isolation
Temporal isolation: when timing prevents reproduction between populations.
Some members of a population will court at different times if there is a lot of competition.
Reproductive periods can change to a different time of day or season. This can lead to speciation.
Example page 346 Monterey Pine vs Bishop Pine